Meanwhile, President Jonathan said an Israeli counter-terrorism team would arrive in Nigeria to help in searching for the schoolgirls, who were abducted last month.

'Slaves'

French troops entered Mali last year to push out al-Qaeda affiliated militants.

Both the US and UK distanced themselves from suggestions that they would send soldiers to take part in the military operation in the vast north of Nigeria.

"There's no intention at this point to be putting any American boots on the ground" said US Defence Secretary Chuck Hagel on Sunday.

UK Prime Minister David Cameron said it was unlikely Nigeria would ask for British troops to help, but he added: "I said to President Jonathan where we can help, please ask, and we will see what we can do."

"I rang the Nigerian president to offer anything that would be helpful and we agreed to send out a team that includes some counter-terrorism and intelligence experts to work alongside the bigger American team that's going out there."

Mr Cameron later tweeted his support for a hashtag aimed at raising awareness of the abductions.

The US First Lady Michelle Obama has described herself and President Barack Obama as being "outraged and heartbroken" over the girls' abduction.

Speaking instead of her husband in the weekly presidential address, she said: "What happened in Nigeria was not an isolated incident. It's a story we see every day as girls around the world risk their lives to pursue their ambitions."

Boko Haram has admitted capturing the girls, saying they should never have been in school and should get married instead. The group has also threatened to sell the girls as "slaves".

Boko Haram has been engaged in a violent campaign against the Nigerian government since 2009.

It is thought the majority of the girls are Christians, although a number of Muslims are among those who were taken.

Chibok, from where they were abducted, is a small community where families are made up of members of both faiths.